MEMS device and process

ABSTRACT

A MEMS capacitive transducer with increased robustness and resilience to acoustic shock. The transducer structure includes a flexible membrane supported between a first volume and a second volume, and at least one variable vent structure in communication with at least one of the first and second volumes. The variable vent structure includes at least one moveable portion which is moveable in response to a pressure differential across the moveable portion so as to vary the size of a flow path through the vent structure. The variable vent may be formed through the membrane and the moveable portion may be a part of the membrane, defined by one or more channels, that is deflectable away from the surface of the membrane. The variable vent is preferably closed in the normal range of pressure differentials but opens at high pressure differentials to provide more rapid equalisation of the air volumes above and below the membrane.

This is a continuation of application Ser. No. 15/470,107, filed Mar.27, 2017, which is a continuation of application Ser. No. 15/290,361,filed on Oct. 11, 2016, which is a continuation of application Ser. No.14/959,737, filed on Dec. 4, 2015, now U.S. Pat. No. 9,487,389, which isa continuation of application Ser. No. 14/636,793, filed on Mar. 3,2015, now U.S. Pat. No. 9,206,031, which is a continuation ofapplication Ser. No. 14/176,811, filed on Feb. 10, 2014, now U.S. Pat.No. 8,987,844, which is a continuation of application Ser. No.13/974,797, filed on Aug. 23, 2013, now U.S. Pat. No. 8,737,171, whichclaims the benefit of Provisional Application No. 61/704,824, filed onSep. 24, 2012 and Provisional Application No. 61/725,380, filed on Nov.12, 2012, the disclosures of which are incorporated herein by referencein their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a micro-electro-mechanical system (MEMS)device and process, and in particular to a MEMS device and processrelating to a transducer, for example a capacitive microphone.

2. Description of the Related Art

Various MEMS devices are becoming increasingly popular. MEMStransducers, and especially MEMS capacitive microphones, areincreasingly being used in portable electronic devices such as mobiletelephones and portable computing devices.

Microphone devices formed using MEMS fabrication processes typicallycomprise one or more membranes with electrodes for read-out/drivedeposited on the membranes and/or a substrate. In the case of MEMSpressure sensors and microphones, the read out is usually accomplishedby measuring the capacitance between the electrodes. In the case ofoutput transducers, the membrane is moved by electrostatic forcesgenerated by varying a potential difference applied across theelectrodes.

FIGS. 1a and 1b show a schematic diagram and a perspective view,respectively, of a known capacitive MEMS microphone device 100. Thecapacitive microphone device 100 comprises a membrane layer 101 whichforms a flexible membrane which is free to move in response to pressuredifferences generated by sound waves. A first electrode 102 ismechanically coupled to the flexible membrane, and together they form afirst capacitive plate of the capacitive microphone device. A secondelectrode 103 is mechanically coupled to a generally rigid structurallayer or back-plate 104, which together form a second capacitive plateof the capacitive microphone device. In the example shown in FIG. 1a thesecond electrode 103 is embedded within the back-plate structure 104.The capacitive microphone is formed on a substrate 105, for example asilicon wafer which may have upper and lower oxide layers 106, 107formed thereon. A cavity 108 in the substrate and in any overlyinglayers (hereinafter referred to as a substrate cavity) is provided belowthe membrane, and may be formed using a “back-etch” through thesubstrate 105. The substrate cavity 108 connects to a first cavity 109located directly below the membrane. These cavities 108 and 109 maycollectively provide an acoustic volume thus allowing movement of themembrane in response to an acoustic stimulus. Interposed between thefirst and second electrodes 102 and 103 is a second cavity 110.

The first cavity 109 may be formed using a first sacrificial layerduring the fabrication process, i.e. using a material to define thefirst cavity which can subsequently be removed, and depositing themembrane layer 101 over the first sacrificial material. Formation of thefirst cavity 109 using a sacrificial layer means that the etching of thesubstrate cavity 108 does not play any part in defining the diameter ofthe membrane. Instead, the diameter of the membrane is defined by thediameter of the first cavity 109 (which in turn is defined by thediameter of the first sacrificial layer) in combination with thediameter of the second cavity 110 (which in turn may be defined by thediameter of a second sacrificial layer). The diameter of the firstcavity 109 formed using the first sacrificial layer can be controlledmore accurately than the diameter of a back-etch process performed usinga wet-etch or a dry-etch. Etching the substrate cavity 108 willtherefore define an opening in the surface of the substrate underlyingthe membrane 101.

A plurality of holes, hereinafter referred to as bleed holes 111,connect the first cavity 109 and the second cavity 110.

As mentioned the membrane may be formed by depositing at least onemembrane layer 101 over a first sacrificial material. In this way thematerial of the membrane layer(s) may extend into the supportingstructure, i.e. the side walls, supporting the membrane. The membraneand back-plate layer may be formed from substantially the same materialas one another, for instance both the membrane and back-plate may beformed by depositing silicon nitride layers. The membrane layer may bedimensioned to have the required flexibility whereas the back-plate maybe deposited to be a thicker and therefore more rigid structure.Additionally various other material layers could be used in forming theback-plate 104 to control the properties thereof. The use of a siliconnitride material system is advantageous in many ways, although othermaterials may be used, for instance MEMS transducers using polysiliconmembranes are known.

In some applications, the microphone may be arranged in use such thatincident sound is received via the back-plate. In such instances afurther plurality of holes, hereinafter referred to as acoustic holes112, are arranged in the back-plate 104 so as to allow free movement ofair molecules, such that the sound waves can enter the second cavity110. The first and second cavities 109 and 110 in association with thesubstrate cavity 108 allow the membrane 101 to move in response to thesound waves entering via the acoustic holes 112 in the back-plate 104.In such instances the substrate cavity 108 is conventionally termed a“back volume”, and it may be substantially sealed.

In other applications, the microphone may be arranged so that sound maybe received via the substrate cavity 108 in use. In such applicationsthe back-plate 104 is typically still provided with a plurality of holesto allow air to freely move between the second cavity and a furthervolume above the back-plate.

It should also be noted that whilst FIG. 1 shows the back-plate 104being supported on the opposite side of the membrane to the substrate105, arrangements are known where the back-plate 104 is formed closestto the substrate with the membrane layer 101 supported above it.

In use, in response to a sound wave corresponding to a pressure waveincident on the microphone, the membrane is deformed slightly from itsequilibrium position. The distance between the lower electrode 102 andthe upper electrode 103 is correspondingly altered, giving rise to achange in capacitance between the two electrodes that is subsequentlydetected by electronic circuitry (not shown). The bleed holes allow thepressure in the first and second cavities to equalise over a relativelylong timescales (in acoustic frequency terms) which reduces the effectof low frequency pressure variations, e.g. arising from temperaturevariations and the like, but without impacting on sensitivity at thedesired acoustic frequencies.

The transducer shown in FIG. 1 is illustrated with substantiallyvertical side walls supporting the membrane layer 101 in spaced relationfrom the back-plate 104. Given the nature of the deposition process thiscan lead to a high stress concentration at the corners formed in thematerial layer that forms the membrane. Sloped or slanted side walls maybe used to reduce the stress concentration. Additionally oralternatively it is known to include a number of support structures suchas columns to help support the membrane in a way which reduces stressconcentration as illustrated in FIGS. 2a and 2b . FIGS. 2a and 2billustrate the periphery of a MEMS microphone structure in perspectiveand cross sectional views respectively, where similar components areidentified by the same numerals as used in FIG. 1.

In this example the MEMS device 200 is formed with a plurality ofsupport structures 201, which in this example are formed as supportingcolumns, arranged around the periphery of the membrane. The columns areformed by patterning the first sacrificial material used to define thefirst cavity 109 such that the substrate 105 is exposed in a number ofareas before depositing the material forming the membrane layer 101(FIG. 2b shows one membrane layer being deposited directly on thesubstrate but it will be appreciated that that there may be variousintermediate layers on the substrate and the membrane may be formed bydepositing multiple membrane layers). Likewise the second sacrificialmaterial used to define the second cavity 110 is patterned so thatmembrane layer 101 is exposed in the same areas prior to depositing thematerial of the back-plate layer. This results in a plurality of columnsbeing formed around the periphery of the membrane which provide supportto the membrane but with a reduced stress concentration compared to thearrangement shown in FIG. 1. The columns are preferably formed with astepped profile and/or slanted side walls to minimise stress. Thisprocess can lead to dimples in the upper surface of the back-plate layerin the area of the columns.

MEMS transducers such as those shown in FIGS. 1 and 2 may usefully beused in a range of devices, including portable devices. Especially whenused for portable devices it is desirable that the MEMS transducers aresufficiently rugged to survive expected handling and use of the device.There is therefore a general desire to improve the resilience of MEMSdevices.

SUMMARY OF THE INVENTION

The present invention is therefore concerned with improving therobustness and/or resilience of MEMS devices.

Thus according to an aspect of the present invention there is provided aMEMS transducer comprising a flexible membrane and at least one variablevent structure wherein the variable vent structure provides a flow pathhaving a size that varies with pressure differential across themembrane.

The variable vent structure may comprise at least one moveable portionwhich is moveable in response to a pressure differential across themoveable portion so as to vary the size of a flow path through the ventstructure.

The equilibrium position of the at least one moveable portion maycorrespond to a minimum size of flow path. The equilibrium position maycorrespond to the flow path being substantially closed.

The flexible membrane may be supported between a first volume and asecond volume and the flow path may be between the first and secondvolumes. At least one variable vent structure may be formed in theflexible membrane and the flow path is a path through the membrane. Theat least one moveable portion may be moveable to expose a hole in themembrane and may comprise a portion of the membrane which is able to bedeflected away from the surface of the rest of the membrane. Themoveable portion of the membrane may be defined by one or more channelsrunning through the membrane. At least one the moveable portions may begenerally triangular, circular or rectangular in shape. In someinstances the moveable portion may be connected to the rest of themembrane via a beam structure. The beam structure may be able to twistto allow said moveable portion to be deflected away from the surface ofthe rest of the membrane.

The beam structure may have a non-rectilinear path, i.e. a tortuouspath, for instance at least part of the beam structure may have aserpentine path or the beam structure may comprise one or more bends,such as right angled bends, within the plane of the beam. The beamstructure may comprise at least one torsional spring located between themoveable portion and the rest of the membrane. The moveable portion maytherefore be connected to the rest of the membrane via a spring, i.e.damping, structure, and the spring structure may be able to twist toallow said moveable portion to be deflected away from the surface of therest of the membrane.

The beam structure may additionally or alternatively be able to bend toallow the moveable portion to be deflected away from the surface of therest of the membrane, e.g. in a direction substantially normal to themembrane. The beam structure may comprise a leaf spring and/or have aserpentine path.

At least one variable vent structure may comprise at least two moveableportions, the at least two moveable portions being able to be deflectedaway from the surface of the rest of the membrane to expose a hole inthe membrane.

In some embodiments the moveable portion may comprise the part of themembrane having said hole in the membrane, the membrane being moveablerelative to a fixed plug portion. The fixed plug portion may lie inplane with the membrane in its equilibrium position and may be supportedrelative to the transducer structure. The plug portion may be supportedfrom the substrate and the substrate may have a channel through thesubstrate in the vicinity of the support for the plug section or theplug portion may be supported from the back-plate. The plug portion maybe formed from the same material as the membrane and/or may be thickerthan the membrane.

At least one variable vent structure may be formed with a flow-path thatbypasses the membrane. The flow-path may run through at least part of asidewall of said transducer structure.

At least one variable vent may have a flow path from one of the firstand/or second volumes to outside the first and/or second volumes.

The variable vent may be configured such that, at pressure differentialsbelow a first threshold, the moveable portion is not completelydeflected out of the surface of the rest of membrane. There may besubstantially no movement of the moveable portion from the equilibriumposition at pressure differentials below a first threshold. The firstthreshold may be greater than 150 Pa and may be greater than 1 kPa. Thevariable vent may provide substantially no significant variation in flowpath size for pressure differentials in the range of 0 Pa-200 Pa.

The variable vent may provide a size of flow path through the vent thathas a non-linear relationship to the pressure differential across themoveable portion.

The at least one moveable portion may be configured such that there issubstantial movement of the moveable portion from the equilibriumposition at pressure differentials above a second threshold. The secondthreshold may be lower than 100 kPa. The variable vent may providesubstantially a significant increase in flow path size for pressuredifferentials in the range of 100 kPa-200 kPa, compared to the flow pathsize at equilibrium. The at least one moveable portion may be moveablein response to a pressure differential across the moveable portion of atleast 100 kPa.

The transducer may comprise a back-plate structure wherein the flexiblemembrane layer is supported with respect to said back-plate structure.The back-plate structure may comprises a plurality of holes through theback-plate structure. When at least one variable vent structure isformed in the flexible membrane layer at least one of the holes throughthe back-plate structure may comprise a vent hole in a location thatcorresponds to the location of a variable vent structure in the flexiblemembrane layer. The area of the vent hole in the back-plate may extendlaterally away from the area of opening of the vent in the flexiblemembrane at a position where the variable vent in the flexible membranefirst opens. When at least one variable vent structure is formed in theflexible membrane layer and comprises a moveable portion which isconnected to the rest of the membrane via a beam structure and themoveable portion and beam structure are defined by channels runningthrough the flexible membrane; then the location of the channels in themembrane which do not form part of the variable flow path through themembrane in use may be arranged so as to not substantially overlap withthe location of any of said plurality of holes in the back-platestructure.

The transducer may be a capacitive sensor such as a microphone. Thetransducer may comprise readout, i.e. amplification, circuitry. Thetransducer may be located within a package having a sound port, i.e. anacoustic port. The transducer may be implemented in an electronic devicewhich may be at least one of: a portable device; a battery powereddevice; an audio device; a computing device; a communications device; apersonal media player; a mobile telephone; a tablet device; a gamesdevice; and a voice controlled device.

In a further aspect the invention provides a method of fabricating aMEMS transducer having a flexible membrane, the method comprising:

-   -   forming a structure having a flexible membrane supported between        a first volume and a second volume; and    -   forming at least one variable vent structure in communication        with at least one of said first and second volumes,    -   said variable vent structure comprising at least one moveable        portion which is moveable in response to a pressure differential        across the moveable portion so as to vary the size of a flow        path through the vent structure.

The method may be used to form transducer according to any of theembodiments discussed above. In particular the method may compriseforming a membrane layer to form at least part of said flexible membraneand forming at least variable vent structure in said membrane layer.Forming the variable vent structure may comprise forming one or morechannels through the membrane so that a portion of the membrane can bedeflected away from the surface of the rest of the membrane in responseto a pressure differential.

In a further aspect of the invention there is provided a MEMS transducercomprising: a transducer structure comprising a flexible membranesupported between a first volume and a second volume; wherein

-   said transducer structure comprises at least one variable vent    structure,-   said variable vent structure comprising at least one moveable    portion which is moveable in response to a high pressure    differential across the moveable portion so as to provide a flow    path for venting gas from at least one of said first and second    volumes.

In a further aspect there is provided a MEMS transducer comprising:

-   a flexible membrane, and-   at least one variable vent structure which is substantially closed    in a first range of pressure differentials and which opens in a    second higher range of pressure differentials to reduce the pressure    differential across the membrane.

The invention, in another aspect, provides a MEMS transducer comprising:

-   a flexible membrane supported between a first volume and a second    volume-   a vent structure connecting said first and second volumes;-   wherein said vent provides a flow path having a size that varies    with pressure differential across the membrane.

In a further aspect there is a MEMS transducer comprising:

-   a flexible membrane supported between a first volume and a second    volume-   a vent connecting said first and second volumes wherein the vent is    configured such that the flow rate through the vent is non-linear    with respect to pressure difference.

In another aspect there is provided a MEMS transducer having a membranesupported between first and second volumes wherein the acousticimpedance between the first and second volumes is variable with thedifferential pressure between the volumes.

Embodiments of the invention relate to a MEMS transducer comprising:

-   -   a transducer structure comprising a flexible membrane supported        between a first volume and a second volume; wherein    -   said transducer structure comprises at least one variable vent        structure for varying the size of a flow path between said first        and second volumes,    -   said variable vent structure comprising at least one moveable        portion which is moveable with respect to a surface, wherein the        moveable portion is connected to the rest of the surface by at        least one torsional spring. The variable vent may be formed in        said flexible membrane.

A further aspect provides a MEMS transducer comprising:

-   a transducer structure comprising a flexible membrane, the membrane    being supported between a first volume and a second volume; wherein-   said transducer structure comprises at least one variable vent    structure in communication with at least one of said first and    second volumes,-   said variable vent structure comprising at least one moveable    portion which is moveable in response to a pressure differential    across the moveable portion so as to vary the size of a flow path    through the vent structure.

The variable vent may be a variable aperture and thus embodiments of theinvention also provide a MEMS transducer comprising: a flexiblemembrane; and at least one variable aperture for equalising a pressuredifferential across the flexible membrane.

In general there is provided a MEMS transducer that comprises at leastone variable vent. The MEMs transducer may be a capacitive microphone.The transducer may have a flexible membrane and the variable vent may beformed in the flexible membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only, withreference to the accompanying drawings, of which:

FIGS. 1a and 1b illustrate known capacitive MEMS transducers in sectionand cut-away perspective views;

FIGS. 2a and 2b illustrate plan, sectional and perspective views ofanother known capacitive MEMS transducer;

FIGS. 3a and 3b illustrate how a high pressure event may affect themembrane;

FIGS. 4a-4c illustrate a variable vent structure according to anembodiment of the invention;

FIG. 5 illustrates a plot of acoustic conductance against differentialpressure and the degree of opening of the vent structure;

FIGS. 6a and 6b illustrate the membrane of a transducer having avariable vent;

FIG. 7 illustrates other embodiments suitable variable vent structures;

FIGS. 8a to 8f illustrate further suitable vent structures;

FIGS. 9a-9c illustrate further examples of suitable vent structures;

FIGS. 10a and 10b illustrates plan views of a membrane having aplurality of variable vent structures;

FIG. 11 illustrates a transducer with a variable vent in a flow paththat by-passes the membrane;

FIGS. 12a-12c illustrate a further variable vent structure according toan embodiment of the invention;

FIG. 13 illustrates another embodiment of a variable vent structure; and

FIGS. 14a to 14h illustrate various arrangements for packages includinga MEMS transducer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described above MEMS transducers such as shown in FIGS. 1 and 2 maybe usefully employed in a variety of different devices and increasinglyare becoming popular for use in portable electronic devices such asmobile telephones, mobile computing devices and/or personal mediaplayers and the like.

To be useful for use in portable electronic devices such transducersshould be able to survive the expected handling and use of the portabledevice, which may include the device being accidentally dropped.

If a device such as a mobile telephone is subject to a fall, this canresult not only in a mechanical shock due to impact but also a highpressure impulse incident on a MEMS transducer. For example, a mobiletelephone may have a sound/acoustic port for a MEMS microphone on oneface of the device. If the device falls onto that face, some air may becompressed by the falling device and forced into the sound port. Thismay result in a high pressure impulse incident on the transducer. It hasbeen found that in conventional MEMS transducers of the form describedabove high pressure impulses can potentially lead to damage of thetransducer.

Referring again FIGS. 2a and 2b , as previously described, a MEMStransducer 200 may have a membrane layer 101 and a back-plate layer 104formed so that a membrane is suspended above a surface of a substrate105 to define a first cavity 109 and the back-plate 104 is suspendedabove the membrane to form a second cavity 110. Note as use herein theterm substrate will be used to refer to the one or more layers ofmaterial above which the membrane is suspended. This may typicallycomprise a silicon wafer and may also include one or more depositedlayers, possibly including layers of the same material used to form themembrane layer.

As mentioned above a sacrificial material may be used to define thedimensions of the first cavity and hence the dimensions of the membrane.As discussed sacrificial material may be deposited and patternedrelatively accurately to provide good control over the membranedimensions. A substrate cavity is also provided in the substrate 105,typically by means of a back etch. To ensure that it is the dimensionsof the first cavity 109 which determine the membrane dimensions, thesubstrate cavity is arranged to have a smaller diameter than the firstcavity at the point 202 where the substrate cavity and first cavitymeet, in other words the opening of the substrate cavity at the surfaceof the substrate has a smaller diameter than the first cavity. Thismeans that in such a structure the membrane is suspended above a sectionof the substrate, indicated by arrow 203, before reaching the opening ofthe substrate cavity, i.e. the opening of the substrate cavity 108 inthe surface of the substrate is within the area of the flexiblemembrane.

The sacrificial material used to define the first and second cavities isdimensioned so as to provide a desired equilibrium separation betweenthe membrane layer 101 and the substrate 105 and also between themembrane layer 101 and the back-plate 104 so as to provide goodsensitivity and dynamic range in use. In normal operation the membranemay deform within the volume defined by the first and second cavitieswithout contacting the back-plate and/or substrate 105.

In response to a high pressure impulse however the membrane layer 101may exhibit a greater amount of deformation than usual. FIG. 3aillustrates the situation where the membrane has been deformed downwardsfollowing a high pressure event and FIG. 3b shows the situation wherethe membrane has been displaced upwards.

Consider the situation where the microphone is arranged to receiveincident sound from a sound port arranged above the back-plate 104 andthe sound port pressure suddenly increases, for instance as a result ofair trapped when the device falls being forced into the sound port. Thismay result the pressure in the second cavity 110 being significantlygreater than the pressure in the first cavity 109, displacing themembrane downwards to greater extent than is usual. This may result in arelatively large stress at point 301 where membrane layer 101 forms partof the sidewall of supporting structure 201 and, in some instances, maythus result in delamination of the membrane layer from the rest of thesidewall structure. Further, if the pressure difference is great enoughthe membrane may make contact with the substrate 105 at the edge of thesubstrate defined by the side wall 202 of the opening of substratecavity 108. Typically the edge of the substrate at the location of theopening of substrate cavity has a relatively sharp angle and thus themembrane may be deformed round this edge, leading to a large stressconcentration at this point 302.

As mentioned previously the membrane layer 101 will typically be formedfrom one or more thin layers of semiconductor material, such as siliconnitride. Whilst such a material can be flexible when subject to evenstresses if there is a significant localised out-of-plane stress, suchas may be introduced into the membrane at point 302 by contact with theedge of the opening of substrate cavity 108, the membrane material canbe relatively brittle. Thus contact between the membrane and the edge ofthe opening of substrate cavity in this way can lead to damage such ascracking of the membrane.

The bleed holes (not shown in FIG. 2 or 3) discussed above with relationto FIG. 1 will provide a flow path between the first and second cavitiesand thus flow of air through the bleed holes will reduce the pressuredifferential acting on the membrane over time. However the bleed holesare typically deliberately arranged to provide a limited amount of flowso as to provide a desired frequency response. Thus a high pressuredifferential may be maintained across the membrane for a relatively longperiod of time before flow through the bleed holes acts to equalise thepressures in the first and second cavities. The time taken to equalisevia the bleed holes could be changed by altering the size and/or numberof bleed hole but this may impact negatively on transducer performance.

As the high pressure caused by trapped air may persist for a relativelylong time, the pressure in the first and second cavities may equalise byvirtue of the bleed holes as discussed. Thus the pressure in the firstcavity, and substrate cavity, may increase until the pressures areequalized. However once air is no longer being forced into the soundport the pressure in the sound port will reduce quite quickly and, astypically the back-plate has a low acoustic impedance, the pressure inthe second cavity will quickly reduce. At this point the pressure in thefirst cavity may be significantly greater than the pressure in thesecond cavity and thus the membrane may be deformed upwards, again to agreater extent than may usually be in the case. Again this may lead to asignificant stress in region 301 where the membrane layer 101 meets thesidewall of the supporting structure. If the pressure difference islarge enough the membrane may be displaced far enough to contact theback-plate 104. This may limit the amount of travel of the membrane ascompared with the situation shown in FIG. 3a but again this mayintroduce stress into the membrane layer at the point 303 where itcontacts the back-plate 104. Again it may take a while for this pressuredifferential to reduce by virtue of flow through the bleed holes.

It should be appreciated that both of these situations can also occurwhen sound is received via the substrate cavity 108 but in the oppositeorder. Whilst both situations may lead to damage of the membrane it isbelieved that the situation shown in FIG. 3a is more likely to lead todamage.

Embodiments of the present invention relate to MEMS transducerscomprising a transducer structure comprising a flexible membranesupported between a first volume and a second volume. The first volumemay for instance comprise the first cavity (109) between the membraneand the substrate and/or the volume formed in the substrate (108). Thesecond volume may comprise the second cavity (110) between the membraneand back-plate and/or any volume in fluid communication with the secondcavity (e.g. a sound port in top-port embodiments). To reduce thelikelihood of damage in high pressure situations the transducerstructure comprises at least one variable vent structure incommunication with at least one of said first and second volumes. Thevariable vent structure comprises at least one moveable portion which ismoveable in response to a pressure differential across the moveableportion so as to vary the size of a flow path through the ventstructure.

The variable vent structure may comprise a moveable portion which ismoveable so as to open a hole extending from the first volume to thesecond volume. The moveable portion may quiescently occupy at leastsome, and possibly most, of the area of the hole, but is moveable inresponse to a local pressure differential across the hole so as to varythe size of the hole which is open to provide a flow path. In otherwords the moveable portion may, in equilibrium, effectively close atleast part of the hole, but is moveable so as to vary to degree to whichthe hole is closed. The moveable portion is preferably arranged toremain closing the hole, i.e. aperture, at normal operating pressuredifferentials but to more to increase the size of the flow path, e.g.close less of the hole, at higher pressure differentials that couldpotentially cause damage to the membrane. The vent can therefore be seenas a variable aperture.

The variable vent structure thus acts as a type of pressure relief valveto reduce the pressure differential acting on the membrane at relativelyhigh pressure differentials. However unlike, the bleed holes in themembrane (if present) which have a fixed area and thus a fixed size offlow path, the variable vent has a flow path size, i.e. aperture, whichvaries in response to a pressure differential. Thus the degree to whichthe variable vent allows venting depends on the pressure differentialacting on the vent—which clearly depends on the pressure of at least oneof the first and second volumes. The variable vent therefore provides avariable acoustic impedance.

Conveniently the variable vent is arranged to provide a greater degreeof venting at higher pressures. Thus the equilibrium position of themoveable portion, i.e. the position the moveable portion adopts whenthere is no substantial pressure differential, corresponds to a minimumsize of flow path. The equilibrium position of the moveable portion maycorrespond to the flow path being substantially closed. Thus atrelatively low pressure differentials, such as may be experienced in theexpected normal operating range of the transducer, the variable vent maybe effectively closed and/or allow only a limited amount of venting.However in a high pressure situation the moveable portion of thevariable vent may move to a more open position, to provide a larger sizeof flow path and thus provide greater venting. This may reduce thepressure differential acting on the membrane and hence reduce the chanceof damage to the membrane.

In some embodiments the flow path of at least one variable ventstructure is between the first and second volumes. The variable vent maycomprise (at least when open) a hole in a structure, the hole connectingthe first and second volumes. Thus the variable vent may allow thepressure in the two volumes to equalise in the event of a high pressuredifferential between the two volumes. The variable vent mayprogressively open at higher pressure differentials to allow more rapidequalisation than would be the case without the variable vent. At lowerpressure differentials the variable vent may provide a minimum flow pathso as to not impact on device performance.

FIGS. 4a to 4c illustrate one embodiment of the invention. In thisembodiment at least one variable vent structure is formed in theflexible membrane 101 and the flow path is a path through the membrane.In other words the variable vent structure may comprise a hole throughthe substrate and the moveable portion of the variable vent structureprovides a variable degree of blocking of the hole, subject to the localpressure differential.

FIG. 4a illustrates a plan view of the variable vent structure 401. Thevariable vent structure comprises part of the membrane 101 formed as amoveable portion 402. In this embodiment the moveable portion 402 isformed as a moveable flap portion. The moveable flap portion 402 isdefined by a channel 403 which runs through the membrane. The channel403, which may be formed by etching through the membrane, is a thinchannel and separates the moveable flap portion 402 partially from therest of the membrane. The moveable flap portion remains attached to therest of the membrane via a connecting portion 404.

Etching channels to partially separate the moveable portion 402 from therest of the membrane in this way means that the moveable portion of themembrane may be deflected away from the surface of the rest of themembrane.

The moveable portion is preferably arranged such that its equilibriumposition, i.e. the position it adopts with substantially no pressuredifferential acting on the moveable portion, is within the plane of themembrane. In other words the moveable portion is not substantiallydeflected away from the rest of the membrane at equilibrium. In thisposition the moveable portion 402 substantially covers the flow paththrough the membrane, i.e. the flow path is at a minimum size and inthis embodiment is substantially closed.

It will of course be appreciated that the channel 403 does represent apath for air to flow through the membrane, however the channel 403 maybe formed with a very narrow width and thus there may be no or limitedair flow through the channel when the moveable flap portion is in theclosed position.

The width of channel 403 may be limited by the photolithographic processconstraints on the minimum etchable gap, or the need for some mechanicalclearance for the moveable element(s) to bend and flex yet clear therest of the structure. Also narrow gaps will tend to have a largerfractional manufacturing tolerance, leading to a wider variation in theacoustic impedance when closed and thus a wider variation in of e.g. thelow-frequency roll-off a microphone.

A typical width might be 1 μm, relative to a typical vent structure of20 μm to 50 μm in extent. However the width might be ten times smalleror larger depending on the acoustic specifications or the manufacturingprocess capability. As mentioned the line width of the channels definingthe moveable vent part may influence factors such as the low-frequencyroll-off. In choosing appropriate line widths the effect of differentwidths may be simulated and/or different designs could be fabricated andtested.

At high pressure differentials the moveable portion may be deflected outof the membrane surface and thus effectively open the flow path throughthe membrane. FIG. 4b illustrates in perspective view the part of themembrane and the variable vent. In this example the pressure in thevolume below the membrane is sufficiently greater than the pressure inthe volume above the membrane such that the moveable flap portion 402has been deflected upwards away from the rest of the membrane surface.This opens the flow channel through the membrane, i.e. effectively opensa hole in the substrate. If the pressure differential increases enoughthe moveable portion 402 may be further deflected and thus provide agreater amount of opening, i.e. a greater flow path.

The moveable portion may thus adopt a range of positions. Thesepositions depend on the pressure differential acting on the moveableportion (or the variable vent). The extent to which the moveable portionis deflected also determines how much the moveable portionblocks/exposes the hole through the membrane and thus the size of theflow path, as illustrated in FIG. 5. FIG. 5 illustrates a graph ofacoustic conductance against differential pressure. The acousticconductance represents how readily air may flow between the two volumesand thus is related to the extent of opening of the flow path, i.e. theextent to which the hole through the membrane is exposed, againstpressure differential.

FIG. 5 also illustrates the relative position of the moveable portion atfour particular pressure differentials, indicated a, b, c and d. At lowpressure differentials/equilibrium the moveable portion lies within thesurface of the membrane and thus the flow path is substantially closed,as indicated by position a—with only the size of the channels definingthe moveable portion providing a flow path. Thus the acousticconductance is low, or substantially zero if the channels defining themoveable portion are thin enough.

At slightly higher pressure differentials the moveable portion may beslightly deflected but still lies at least partly within the planesdefined by the upper and lower surfaces of the membrane. Thus the flowpath remains substantially closed. Position b illustrates the positionwhere the moveable portion is deflected upwards and the bottom part ofthe moveable portion is just about extending beyond the top surface ofthe membrane. Thus the acoustic conductance is still very low.

At higher pressure differentials the moveable portion is deflected sothat at least part of the moveable portion extends completely beyond themembrane surface. This provides a certain size of flow path—however thehole is still partially blocked by the moveable portion. This isrepresented by position c.

As the amount of deflection increases the area of unblocked flow pathincreases until, at position d, the moveable portion is moved completelyout of the area of the flow path and the vent is fully open with adefined maximum area. Even if the pressure differential increasesfurther the size of the flow path will not increase any further. Inpractice however in some embodiments, the moveable portion may not bedeformable to the full extent shown in position d.

It will be appreciated that the acoustic conductance (or acousticimpedance), i.e. the size of the flow path, does not exhibit a linearrelationship with differential pressure. Thus the rate of flow will varydepending on how open the variable vent is. Until the moveable portionis deformed to the full extent, the rate of flow will increase more thanlinearly with applied differential pressure. Also pressure in anyreceiving volume due to an incoming step in applied pressure on theother side of the membrane will show a rise time depending on a timeconstant related to the acoustic capacitance of the receiving volume andthe acoustic conductance of the vent, so the rise time will alsodecrease with increasing pressure steps, tending to reduce the peakpressure difference across the membrane and hence its deformation orstress.

In practice the acoustic pressure relating to even very loud sounds willbe at least a couple of orders of magnitude below the pressure levelsthat the a microphone needs to withstand in accidental fault or overloadconditions described. Thus for normal acoustic pressure levels, the ventstructure will be operating well below pressure b, so will havenegligible effect on e.g. the low-frequency roll-off.

Conveniently the variable vent is arranged so that the vent remainssubstantially closed (e.g. somewhere between positions a and b for theexample shown with a moveable flap) during the pressure differentialsexpected in normal operation of the transducer and only starts to opensignificantly (i.e. extend beyond position b) when the pressuredifferential reaches unusually high levels or starts to approach levelsthat may potential cause damage to the transducer. It will beappreciated that having a flow path through the membrane could alter theoperating characteristics of the transducer. As discussed above inrelation to FIG. 1 in a MEMS microphone there may be one or more bleedholes though the membrane to reduce the impact of low frequency effects.The number and dimensions of these holes are carefully chosen to providea desired operating characteristic. These bleed holes thus alreadyprovide a path for equalising the pressures in the two volumes on eitherside of the membrane, but these holes are deliberately designed suchthat such equalisation takes a long time in acoustic terms. Thus thebleed holes alone do not prevent large pressure differentials fromcausing damage to the transducer. The variable vents are provided toenable more rapid equalisation. However were the variable vents to beopen to provide a significant flow path at the expected normal operatingpressure differentials such additional flow paths would alter thefrequency characteristics of the transducer and could result indistortion.

Thus the moveable portion of the variable vent may be configured suchthat there is substantially no movement of the moveable portion from theequilibrium position at pressure differentials below a first threshold.In particular the moveable portion may be deflected by less than thewidth of the membrane so that the trailing surface of the moveableportion (i.e. the surface on the opposite side to the direction ofdeflection) does not substantially extend beyond the membrane surface.Thus the flow path, i.e. hole through the membrane, remains largelyblocked by the moveable portion. For acoustic transducers and the likethe first threshold may by greater than 150 Pa and may be greater than200 Pa or higher and could, in some applications be greater than 1 kPa.In other words the variable vent may remain substantially closed atpressure differentials up to about 150 Pa-200 Pa or higher. Thus thevariable vent may provide substantially no significant variation in flowpath size for pressure differentials in the range of 0 Pa-200 Pa. Thismeans that the variable vent has minimal performance impact on theoperation of the transducer.

The variable vent is arranged to open to provide a flow path at pressuredifferentials which approach the pressure differentials that may causedamage to the transducer. For instance the variable vent may be arrangedto be open enough to provide a significant flow path for venting at apressure differential of around 100 kPa. Thus the moveable portion maybe configured such that there is substantial movement of the moveableportion from the equilibrium position at pressure differentials above asecond threshold and the second threshold may be lower than 100 kPa.Thus the variable vent provides substantially a significant increase inflow path size for pressure differentials in the range of 100 kPa-200kPa, compared to the flow path size at equilibrium.

The pressure differential at which the variable vent will open willdepend on various factors such as the thickness and composition of thematerial forming the moveable portion, e.g. the membrane and also (for aflap arrangement) the width of the connecting portion 404 compared tothe area of the flap portion 402. For a MEMS microphone transducer witha membrane formed from silicon nitride, the membrane being of the orderof 0.4 μm thick, suitable variable vents may be formed by etching asuitable shape and size of moveable flap portion as described above. Forexample the design shown in FIG. 4a was simulated with a flap portionradius of 12 μm and a connecting portion width of 6 μm. The resultsindicated that the variable vent will remain substantially closed atpressure differentials of up to between 1 kPa and 5 kPa. At between 20kPa-50 KPa the vent is partially open and at 100 kPa the vent is openenough (i.e. such as illustrated in FIG. 4b ) to provide a significantflow path.

Such a vent, when open, provides a significant flow path between thefirst and second volumes and thus significantly increases the rate atwhich pressure equalisation between the first and second volumes occur.This reduces the time for which the membrane may be exposed to a highstress. In addition however the variable vent can reduce the maximum orpeak pressure differential experienced by the membrane.

To explain, imagine a high pressure impulse caused by trapped air beingforced into a sound port due to the host device falling onto a surface.The pressure in the sound port will increase over a certain rise time,i.e. the pressure profile in the sound port will have a certain risetime. Now consider two examples. In the first example such an impulse isexperienced by a conventional MEMS microphone and in the second examplethe pressure impulse is incident on an embodiment according to thepresent invention.

In the first example with the conventional microphone the increased airpressure in the sound port, and hence one of the volumes (say the firstvolume) will increase the flow rate through the bleed holes in themembrane but the size of the bleed holes is fixed. Thus a certain peakpressure differential will be reached which could potentially be of theorder of 800 kPa or more. In the second example, with an embodiment of aMEMS transducer according to the present invention, as the pressuredifferential increases to high levels, e.g. 50 kPa or so, the variablevent may start to open thus providing some additional venting (inaddition to the bleed holes) from the first volume, thus raising thepressure of the second volume towards the same level (and possiblyreducing the pressure in the first volume compared to what it may havebeen). As the pressure in the sound port increases further, say to 100kPa, the variable vent will be open providing a significant flow paththus providing much quicker equalisation. Depending on the nature of thepressure impulse the venting may reduce the peak pressure experienced inthe first volume compared to the first example but in any case thepressure in the second volume will have a faster rise time, thusreducing the peak pressure differential experienced.

FIG. 6 illustrates the operation of the variable vent. FIG. 6aillustrates the flexible membrane 101 of a transducer (the rest of thetransducer structure being omitted for clarity). The membrane issupported between a first volume and a second volume. In this examplethe first volume includes the cavity 109 between the membrane andsubstrate. The first volume may additionally or alternatively comprise acavity in the substrate. The second volume may include the second cavity110 between the membrane and back-plate. The second volume may alsocomprise an area outside of the back-plate which experiences effectivelythe same pressure variations as the second cavity.

The membrane has a plurality of bleed holes 111 which are dimensionedand arranged to produce a tuned effect on the transducer and reduce theimpact of low frequency pressure variations. The membrane is alsoprovided with a plurality of variable vent structures 401 as describedabove. In some applications it may be possible to use a single variablevent structure but in some applications it may be beneficial to providethe membrane with a plurality of variable vent structures. Where thereare a plurality of variable vent structures they may be distributedrelatively evenly around the membrane.

FIG. 6a shows the variable vent structures being located on the membraneoutside of the area of the membrane electrode 103. This means that thevent is formed just of the materials of the one or more layers formingthe membrane 101. However in some embodiments it would be possible toform the variable vent structure within the area of the electrode, forinstance in exclusion areas within the overall area of the electrodewhere no metal electrode is deposited. Alternatively the variable ventstructure could be formed in the area of the electrode with the membraneand electrode layers together forming the variable vent structure. Insome applications forming the moveable portion of the vent from themembrane layer and metal layer may provide a stronger moveable portion.It should be noted that wherever the variable vent structure is formedthere may be one or more additional materials coupled to the membrane atthat location so as to tailor the properties of the variable vent, forexample the flexibility or stress handling capability.

FIG. 6a illustrates the situation in normal operation where the pressurein the second volume 110 is greater than the pressure in the firstvolume. The membrane is thus deflected downwards from the membraneequilibrium position. However the pressure differential is within thenormal expected operating range of the device, i.e. below an operatingthreshold, and thus the variable vents 401 remain substantially closed.

FIG. 6b shows the situation where the pressure differential hasincreased beyond a threshold to a level sufficient to causes thevariable vents to open. The moveable portions of the membrane, which inthis example are moveable flap portions, are thus deflected downwardsthus opening up flow paths, i.e. holes, through the membrane which moreallows more rapid equalisation with the benefits discussed above.

The material of the membrane 101 is relatively resilient. Thus if thepressure in the second cavity stops increasing then, after a short time,the venting through the variable vents 401 will reduce the pressuredifferential to a level at which the variable vents return to the closedposition shown in FIG. 6a . If the pressure in the second cavity werethen to reduce relatively quickly the pressure differential across themembrane in the opposite direction may increase, such that the membraneis deflected upwards. The pressure differential may increase to such anextent that the vents now open in the upward direction to vent air fromthe first volume into the second volume. It will thus be appreciatedthat the variable vents may be bi-directional and allow venting from thefirst volume to the second volume and vice versa.

It will be seen from FIG. 6b that when the variable vent is open themoveable flap portion will be deformed away from the surface of themembrane—in the same direction that the membrane is deflected. Thus themoveable flap portion could potential extend further than the membraneitself. In some embodiments the membrane may be arranged relative to therest of the transducer structure, such as the back-plate or somestructure of the substrate, such that the membrane may make contact withthe transducer structure (illustrated as 601 in FIG. 6b ). In someinstances this may be beneficial in preventing too much travel of themembrane. Clearly the variable vents need to be able to open to providethe advantages described above and thus the variable vents arepreferably arranged with regard to the transducer structure so that thetransducer structure will not prevent the vents from opening. Also itmay be preferred that there is no structure immediately within outletpath for the vent when open. In some instance the vents may be arrangedon a part of the membrane such that the vents will not come into contactwith the transducer structure. For example with regard to the back-platethe vents may be arranged so that the moveable portions open into anarea of one or more of the back-plate acoustic holes. In otherembodiments however the vents may be arranged such that they can opensufficiently to provide a significant flow path but are prevented fromopening any further by the transducer structure. Thus the transducerstructure may act as a hard stop for the moveable portion of the ventwhich may reduce or limit the stress in the moveable portion and helpprevent damage to the moveable portion.

The moveable portion of the variable vent may take many forms. FIG. 7illustrates a variety of different configurations of suitable variablevent structure. In the top left a number of interconnecting channels 702are etched to leave a number of moveable flap portions 701. In thisexample the channels are etched as a cross shape to define fourtriangular moveable flap portions. This configuration provides arelatively wide connecting portion, indicated by dotted line 703,compared to the surface area of the flap portion. Such a configurationmay be relatively strong.

It will be appreciated that when subjected to a pressure differentialwhich is sufficient to cause the moveable flap portion to deform therewill be significant stress on the connecting portion. It is desirablefor the variable vent to be able to survive high pressure differentialwithout damage, thus a wide connecting portion may be preferred in someapplications. It is noted however the action of the vent opening willresult in the effective area of the moveable flap portion exposed to thehigher pressure area being reduced. Thus, for a fixed pressuredifferential, the force on the moveable flap portion will reduce as thevent opens which helps prevent the moveable flap portion from deformingtoo far.

In some instance however the variable vent structure may comprisesmaterial which is provided specifically to ensure desirable propertiesof the vent structure. For instance FIG. 7 also shows a generallycurved, e.g. semi-circular, moveable flap portion 704 formed by channel705 with a layer of strengthening material 706 provided in the vicinityof the connecting region. The strengthening material could for instancecomprise a metal and may for instance be formed of the same materialused to form the membrane electrode although other materials may ofcourse be used.

FIG. 7 also shows a rectangular moveable flap portion 707 formed by achannel 708 around three sides only and a generally square flap portion709 formed by channel 710 and having a relatively narrow connectingportion. In general the moveable flap portion of the membrane is definedby one or more channels running through the membrane and the flapportions may be, for example, one of triangular, circular, elliptical orrectangular in shape or generally have the shape of any suitable regularor irregular polygon.

FIGS. 8a and 8b shows some further examples of suitable moveableportions. FIG. 8a shows a variable vent structure 801 comprising amoveable portion 802 which comprises an irregular polygon shape definedby channel 803. In this embodiment however the channel 803, togetherwith additional channel 804 mean that the moveable portion 802 isconnected to the rest of the membrane by a beam structure 805. The beam805 connects to the rest of the membrane at both ends and supports themoveable portion 802. As with the embodiments described previously apressure differential acting on the membrane, and hence the moveableportion 802, tends to deflect the moveable portion 802 out of the planeof the membrane. In this embodiment however deflection occurs due to atwisting of the beam 805, rather than an out of plane bend of theconnecting portion. The beam 805 thus acts as a torsional beam. Thestress and deflection of moveable portion achievable is controllable viathe dimensions of the torsion beam 805. This can allow the same degreeof deflection as the embodiments discussed previously but with lowerstress and hence less likelihood of damage—or alternatively allow agreater degree of opening for a given pressure differential.

FIG. 8a also shows a further variable vent structure 806 comprising twosemicircular moveable portions 807 a and 807 b defined by channels 808 aand 808 b and also channel 809, connected to the membrane by torsionalbeams 810 a and 810 b. Again movement of the moveable portions 807 a and807 b involves twisting of the beams 810 a and 810 b.

FIG. 8b shows a further variable vent structure 811 in plan view andalso in sectional view when at least partly open. The variable ventstructure 811 comprises two moveable portions 812 a and 812 b, which inthis example have generally trapezoidal shapes defined by channels 813 aand 813 b (including common central channel 813 c), with channels 814 aand 814 b defining torsional beams 815 a and 815 b supporting themoveable portions.

Again the properties of the vent in terms of the pressure differentialat which the vent opens and the stresses on the vent structure can becontrolled by appropriate choice of dimensions, as can theopen-to-closed ratio of the vent. In one example, where the vent isformed in a silicon nitride membrane of the order of 0.4 μm thick, thewidth of channels defining the vent structure, i.e. dimension ‘a’ inFIG. 8b , may be of the order of 1 μm. The width of the beam structure,dimension ‘b’ may b of the order of 3 μm. The moveable portion 812 a and812 b may each be about 15 μm wide, i.e. dimension ‘c’ of the width ofthe moveable portion from central channel 813 c to the beam structure.The length of the beams, dimension ‘d’ may be of the order of 30 μm.

It will be noted that the overall shape of the flow path of the ventstructure shown in FIG. 8b is defined by the two moveable portions 812 aand 812 b and is generally hexagonal. This may be a particularadvantageous shape for the vent.

As mentioned previously the vent structure may be arranged with respectto the other structure of the transducer, such as the back-plate 104, sothat the vent is aligned with a gap in the back-plate to allow the ventto open to a desired extent. Thus, as described the vent may be alignedwith acoustic holes in the back-plate 104. In some embodiments howeverthe size of the acoustic holes typically provided in the back-plate maybe smaller than the size of the vent. Thus in some embodiments thestructure of the back-plate 104 is provided with larger holes 816 orgaps in the vicinity of the vent structures.

The back-plate 104 is typically designed to be relatively acousticallytransparent and thus providing additional holes in the vicinity of thevents is acceptable. Nevertheless changing the size and/or distributionof the holes through the back-plate may have an impact on the acousticproperties of the device, for instance the low-frequency roll-off of amicrophone. The provision of holes in the back-plate to allow the ventsto open may therefore be compensated by a reduction in the size and/orspacing of the acoustic holes 112 to maintained desired propertiesand/or the holes in the back-plate may be closely matched to the shapeof the vents.

FIGS. 8c and 8d illustrate the arrangement of holes through theback-plate in two embodiments for a variable vent of the form shown inFIG. 8b . FIGS. 8c and 8d show a plan view of a part of the back-plateand illustrates a regular array of acoustic holes 112, which in thisexample are arranged in a regular hexagonal pattern. Also shown is thestructure of an underlying variable vent in the membrane and themoveable portions 812 a and 812 b. It can be seen the moveable portionsare larger than the size of the acoustic holes 112. For example theacoustic holes could be of the order of 10 μm in diameter, with eachacoustic hole separated from its nearest neighbour by about 5 μm. In theexample described above the width of each of the two moveable portionswas of the order of 15 μm.

Thus, in this embodiment, in the vicinity of the vent there is a largervent hole 816 in the back-plate 104 to allow space for the moveableportions 812 a and 812 b to open. The vent hole 816 provided in theback-plate may be sized and shaped to match the vent shape. FIG. 8cshows an arrangement where the vent hole 816 corresponds to overlayingthe acoustic hole pattern in the vicinity of the vent with a patterncorresponding to the vent. FIG. 8d shows an alternative arrangementwherein the acoustic holes pattern is omitted in the vicinity of thevent and instead a vent hole 816 corresponding to the shape of the ventstructure is provided.

As illustrated the vent hole in the back-plate may be at least the sizeof the variable vent and may be generally the same shape, although itwill be appreciated that in some embodiments different shapes could beused and the moveable portion of the vent may only require a smalleropening in the back-plate to accommodate the necessary range of movementof the moveable portion.

It will also be seen that in this embodiment the general shape of thevent structure matches the arrangement of the acoustic holes with theresult that the larger vent holes 816 can be readily accommodated in thenormal pattern of the acoustic holes.

In the embodiment shown in 8 c, the vent hole 816 in the back-plateincludes an area 817 which is positioned near where the variable ventwill first open but which is laterally offset from the area of the vent,i.e. which doesn't overlap with the area opened in the membrane when thevent opens. Such an arrangement can be advantageous in maximising theamount of venting through the variable vent at positions where themoveable portions 812 a or 812 b have not been fully deflected. In otherwords providing such an area of back-plate hole which is near to theposition where the vent opens, but which is slightly offset from theposition of the vent, can aid in maximising the amount of venting as thevent is opening and thus ensure that significant venting occurs as soonas possible when a pressure differential sufficient to cause the vent toopen exists.

Referring back to FIG. 8b it can be seen that as the moveable flapportions 812 a and 812 b deflect, so as to provide a flow path throughthe membrane, the size of the flow path in a direction normal to themembrane is effectively defined by the gap opened between the moveableportions 812 a and 812 b. Thus air passing through the vent may beeffectively funnelled through this gap by the moveable flap portions 812a and 812 b. It will be appreciated however that the size of such a gapmay remain relatively small until the moveable portions 812 a and 812 bhave been deflected to a reasonably significant degree.

For example consider that the moveable portions each have the same width(i.e. dimension “c” in FIG. 8b ) so that if both moveable portions weredeflected to be normal to the membrane (assuming this would be possible)then the gap would be have a maximum value G. If the moveable portionswere each deflected to lie at an angle of about 45° to the membrane thenthe gap would be of the order of about 0.3G (neglecting any deformationof the shape of the moveable portion).

As the moveable portion deflects away from the membrane it may also bepossible for air to pass through the exposed path through the membraneand then deflect laterally, i.e. in the example shown in the sectionalview of FIG. 8b air in the region between moveable portion 812 a and theplane of the membrane (i.e. between the moveable portion 812 a and itsillustrated quiescent position) may vent in a direction into or out ofthe page as illustrated.

Whilst this side venting will occur in practice, in the situation wherethe moveable portion is being deflected into the cavity between themembrane and back-plate it has been found that having the vent hole inthe back-plate (which corresponds to the variable vent) extend to anarea where such side venting may occur can be advantageous to increasethe amount of such venting.

FIG. 8e illustrates the situation where a vent having the structureshown generally in FIG. 8b is partially open when the membrane 101 hasbeen deflected towards back-plate 104. It will be appreciated that theback-plate 104 may have a thickness of the order of a few microns ormore. FIG. 8e shows a view looking side on compared with the view shownin FIG. 8b and shows moveable flap portion 812 a in a partiallydeflected position away from membrane 101, towards back-plate 104. Asmentioned the vent hole 816 in the back-plate may be dimensioned to atleast correspond to the size of the variable vent to allow the vent toopen in the area of vent hole 816. In the embodiment shown in FIG. 8ethe vent hole 816 in the back-plate has area 817 which extends laterallyaway from the area of the vent at a position where the vent first opens.This allows a flow path through the vent illustrated by the solid arrowwhich passes through the plane of the membrane but then vents out theside of the partially open vent. The dotted area 818 illustrates whatwould happen in the absence of such lateral area 817 of back-plate hole.Some air could still possibly vent from the side of the partially openvent into the cavity between the membrane and back-plate, if themembrane 101 were not in contact with the back-plate 104 as illustratedby the dashed arrow. The degree to which side venting could occur mayhowever be much reduced without the lateral area 817.

In various embodiments therefore for at least some vent holes in theback-plate which are provided in the location of a variable vent, forexample to allow space for the variable vent to open into the area ofthe vent hole within the back-plate, the area of such a back-plate venthole may extend laterally away from the area of opening of the vent, ata position at or near where the vent first opens, in order to allow orimprove side venting.

As mentioned above the use of torsional beams such as illustrated in theembodiments of FIGS. 8a and 8b above can be advantageous in allowing themoveable flap portion of the variable vent to move to open the ventwhilst limiting the stress within the structure. One factor that impactson the degree of movement of the moveable portion of the variable ventand its related stress is the length of the torsional beam, i.e.dimension d in FIG. 8b . Longer beam lengths allow a greater degree ofmovement for a given stress level or reduces the overall stress for agiven degree of movement. It may therefore be desirable in someembodiments to provide beams of relatively long length.

It will be noted however that use of a variable vent structure in themembrane including a torsional beam involves channels being formed inthe membrane to define the torsional beam. For instance in FIG. 8bchannels 814 a and 814 b are required to partly define the torsionalbeams 815 a and 815 b. These channels do not form part of the variableflow path part of the vent, i.e. such channels do not form part of thevariable flow path which is opened by movement of the moveableportion(s) of the vent. In order to minimise the effect of the ventstructure when the vent is closed it can be beneficial to limit theextent to which such channels are located in the same position as anacoustic hole in the back-plate. If a channel through the membrane islocated at the same position as a hole in the back-plate this canprovide a small flow path even when the vent is closed. For channels inthe membrane which are used to define the moveable portion of the ventstructure, such as channel 813 c, it may be inevitable that suchchannels will correspond to a hole in the back-plate. Such channels willeffectively form part of the vent flow path when open and thus may beprovided in a location that corresponds to the larger back-plate hole816. However it can be beneficial to configure any channels which do notcorrespond to the variable flow path of the vent structure so as tominimise the extent to which they overlap with any back-plate holes,i.e. underlie (or overlie depending on the transducer structure) anyacoustic holes in the back-plate.

In some embodiments therefore the pattern of back-plate holes, i.e.acoustic holes 112, may be arranged such that one or more areas of theback-plate that correspond to the location of channels in the membranethat define part of the variable vent structure are substantially devoidof any back-plate holes.

In other words there may be at least one vent hole in the back-platethat corresponds to the flow path of the variable vent, i.e. large hole816, and which may be sized to allow the vent to at least partly open insuch a hole. Such a vent hole in the back-plate may be sized and shapedto generally correspond to the flow path enabled by the variable ventwhen open—with possible lateral areas at the location where the ventfirst opens to allow early side venting. However the vent hole may bearranged so as to not substantially extend over any channels in themembrane used to define part of the variable vent structure in themembrane that do not form part of the variable flow path in use.Referring to FIG. 8d the hole 816 is arranged so that it does not extendas far of the upper and lower channels used to define the outer edges ofthe torsional beams. The pattern of other holes in the back-plate, i.e.acoustic holes 112 may then be arranged so as also not to substantiallyoverlap with any channels of the vent structure, i.e. to define areaswhere the channels in the membrane may be provided.

In some embodiments however it may be desirable to have a regularpattern of acoustic holes without any area significant devoid ofacoustic holes in the back-plate. Such a pattern of acoustic holes maytherefore limit the maximum length of the channels that can be createdto define the torsion beam structure without significant overlappingwith acoustic holes. For instance, consider the arrangement illustratedin FIG. 8d where the acoustic holes 112 in the back-plate are arrangedas a hexagonal packed array and the variable vent has a generallyhexagonal shape. If the acoustic holes 112 are say of the order of 10 μmin diameter and separated from one another by 5 μm then the maximum gapbetween adjacent ‘rows’ of acoustic holes is less than 3 μm. Asdiscussed above, in one example the beam may be of the order of 3 μmwide with the channels defining the beams being 1 μm wide. It can thenbe seen therefore that in such an arrangement it may not be possible toextend the length of the torsional beam beyond a certain length, in thisexample of the order of about 30 μm, without at least one of thechannels that defines the torsional beam overlapping with an acoustichole in the back-plate or without having to omit some of the acousticholes 112 from the array with a consequent possible impact onperformance.

In some embodiments therefore a beam structure connecting a moveableportion to the rest of the membrane may have a non-rectilinear path. Inother words the path created by the beam connecting the moveable portionto the rest of the membrane may have one or more bends in it within theplane of the beam. This can increase the effective path length of beamthat can be provided within a given distance. For example the beam mayhave a serpentine or meandering structure as illustrated in FIG. 8f . Inthe example illustrated in FIG. 8f channels are formed in the membraneto form two moveable portions 812 a and 812 b as described above withrelation to FIG. 8b . These moveable portions are connected to the restof the membrane via beam structures 819 a and 819 b. In this embodimenthowever the channels defining the beam structures 819 a and 819 b havesections 820 and 821 extending transversely with respect to the overallbeam length to define serpentine sections 822 of the beam structure.These serpentine sections 822 act as spring sections and increase theeffective length of the beam structure. The effective length of the beamstructure is equal to the overall length of the beam structure L plusthe width W of the spring structure for each bend. Thus the beams 819 aand 819 b illustrated in FIG. 8f have an effective length of L+6W. Usingsuch a spring structure to increase the effective length of the beamstructure has the advantages described above, of allowing a greaterdegree of movement for a given stress level/reducing the overall stressfor a given degree of movement but without requiring the overall lengthof the beam structure to be increased.

In the embodiment of FIG. 8f the same spring structure is formed in eachof the two arms of a beam structure connecting a moveable portion to thesubstrate. Such an arrangement may be beneficial in some materialsystems to ensure that the stresses induced by deflection of themoveable portion are evenly distributed—however this need not be thecase and in other embodiment more bends in the beam structure may bepresent in one arm than the other.

FIG. 8f shows right angled bends in the beam path and generally squarecorners. It will be appreciated however that a range of other shapes arepossible. For instance the bends in the beam path may be lower than 90°(or greater in some applications) and/or be more rounded, for instanceto reduce stress.

As mentioned the serpentine type shape, i.e. tortuous path, provides atorsional spring structure and thus in general the moveable portions areconnected to the rest of the membrane via one or more torsional springs.Using torsional springs in this way can reduce the pressure differentialat which the variable vent opens whilst maintaining a low footprint ofthe vent structure (so that the channels forming the spring structure donot need to substantially overlap with any back-plate holes) and lowstress properties.

A vent design with rectilinear torsion beams such as shown in FIG. 8bwas produced and compared to a vent design incorporating a serpentinespring structure such as shown in FIG. 8f . The overall length L of thebeam structures was the same in both cases and the size and shape of themoveable portions were the same. The deflection of the moveable portionsof the vent including the torsional springs was significantly increasedfor a given pressure differential compared to the vent with straighttorsional beams. In one test a pressure differential of 50 kPa led to a12.3 μm deflection in a variable vent with the torsional springscompared to a deflection of 4.5 μm in the vent with a straight torsionalbeam. This results in a factor of 18 reduction in the air resistance ofthe vent at that pressure.

In general therefore in some embodiments the variable vent may comprisea moveable portion which is moveable to expose a flow path through asurface where the moveable portion is connected to the surface by atleast one torsional spring. The torsional spring may comprise a beamdefining a non rectilinear path. The surface may be the membrane and themoveable portion may a portion of membrane material which is moveablewith respect to the rest of the membrane.

FIGS. 9a-9c show yet further examples of suitable variable ventstructures. In these examples the variable vent structure comprises amoveable portion which is moveable out of the plane of the membrane toprovide a flow path. FIG. 9a shows a first variable vent structure 901which has a moveable portion 902 which is defined by channels 903. Thechannels 903 are arranged to define beam portions 905 a and 905 b thatcan bend in response to a pressure differential on the moveable portion902 so that the moveable portion can be deflected out of the surface ofthe membrane, as illustrated in the sectional view in FIG. 9 a.

FIG. 9b shows another vent structure 906 along similar lines having amoveable portion 907 connected to the rest of the surface by beams, butwith additional channels so as to defining a serpentine beam structure908 to provide a greater degree of bending.

FIG. 9c shows a further example of a variable vent structure 909 havinga moveable portion 910 connected by a plurality of beams 911 whicheffectively acts as leaf springs supporting the moveable portion 910.

It will of course be appreciated that the shape of the moveable portionmay vary and could for instance be circular or elliptical or generallyin the shape of a regular or irregular polygon, with varying numbers ofsupporting arms or leaf springs which may or not have bends therein todescribe a serpentine structure. In general then the moveable portionmay be moveable in a direction which is generally normal to the plane ofthe membrane and supported by beam structures that may be straight orcurved or formed as spring structures.

As discussed previously there may be more than one variable ventstructure provided in the membrane and the variable vent structures maybe evenly spaced around the membrane as shown in FIG. 10a whichillustrates a plan view of the membrane 1001 of a transducer and whichillustrates the spacing of variable vents structures 1002 and bleedholes 1003.

Alternatively the vents may be arranged in other patterns such as shownin FIG. 10b . FIG. 10b also shows the orientation of the vent structures1002 may be the same in different locations, for instance to fit intothe pattern of acoustic holes and larger back-plate holes as describedpreviously.

As described above at least one variable vent may be formed in themembrane. Additionally or alternatively at least one variable vent maybe formed with a flow-path that bypasses the membrane. For instance theflow-path may bypass the membrane and run through at least part of asidewall of some transducer structure.

FIG. 11 shows an embodiment having a flow path with a variable vent 1101that bypasses the actual membrane. FIG. 11 shows part of the supportstructure of the transducer including at least a back-plate structure104 and one or more membrane layers 101. In this example the side wallsof back-plate structure 104 and membrane layer(s) 101 are patterned toprovide a first port 1102 of a flow path. In this example port 1102provides a flow path to a volume outside the first cavity 110. As theback-plate structure is relatively acoustically transparent however dueto acoustic holes 112 the pressure in the first cavity is closely linkedto the pressure in this area. Thus the port 1102 provides a flow pathto/from a volume which includes the second cavity 110.

The substrate 105 is also etched to provide a port 1104 for a flow pathto/from a volume comprising the first cavity 109 and substrate cavity108.

Located in the flow path between ports 1102 and 1104 is a layer ofmaterial 1103 deposited on the substrate and formed to include avariable vent 1101. The variable vent may have the form of any of thevents described above (with the vent being formed in the layer 1103rather than the membrane as described above). In this instance thematerial of the vent may chosen for desired properties without resultingin a change in the properties of the membrane. The thickness of thelayer may also be controlled to provide desired vent characteristics.However material layer 1103 may be any material layer which is providedas part of the transducer structure and could itself be the samematerial as the membrane. Operation of the variable vent is as describedpreviously. A pressure differential across the membrane with also leadto a similar pressure differential across the variable vent. At pressuredifferential encountered in normal operation the variable vent may beremained closed and thus the acoustic properties of the transducerdepend on the membrane. If a high pressure differential is encounteredthe vent may open helping to equalise the two volumes and thus reducethe pressure differential on the membrane.

The embodiments discussed above have focussed on a flow path connectingthe first volume to the second volume. Such an arrangement isadvantageous as it increases the pressure in the low pressure volume aswell as potentially reducing the pressure in the high pressure volume.However in some applications it may be possible to provide a variablevent that has a flow path from one of the first and/or second volumes tooutside the first and/or second volumes. In other words rather thanconnect the first volume directly to the second volume, the flow pathmay be to some other volume. The other volume may be a locally enclosedvolume, and may be contained in the transducer or its package or adevice containing the transducer, or may be a volume not locallyenclosed including even the outside atmosphere. In normal operation sucha variable vent will remain closed but in response to a high pressure inthe relevant volume the vent may open to provide venting from the highpressure volume to help reduce the absolute pressure. Alternatively forthe volume which is not directly connected to or part of the sound port,e.g. a back-volume, the vent could open in response to high pressure inthe outside atmosphere so that the pressure in the back-volume increaseat the same time as the pressure in sound port is increasing thepressure in the other volume.

FIG. 12 illustrates a further example of a variable vent according to anembodiment of the invention. FIG. 12a illustrates the membrane 101suspended from substrate 105 and back-plate 104 as described previously.In this embodiment however there is a hole in the membrane 101 which, atequilibrium position, is substantially closed by a plug section 1201.The plug section 1201 is supported so as to be substantially fixed inposition In other words the membrane 101 is moveable with respect to theplug section 1201 with the plug section being substantially fixed withrespect to the membrane support structure, i.e. the substrate 105,back-plate 104 and/or side wall structures. In the example shown in FIG.12a the plug section is supported from the substrate 101 in the areawhere the membrane overlies the substrate 105 by one or more supportstructures 1202 which may for example be support columns. The supportstructure keeps the plug section in position.

In use in equilibrium the plug section 1201 substantially blocks theholes in the membrane 101 and thus prevents a substantial flow of air asdescribed previously. as the membrane deflects away from equilibriumposition the plug remains in position and thus increasingly the holethrough the membrane is exposed, providing an increased flow path asdescribed previously.

FIG. 12b shows the situation with the membrane deflected downwards (withthe backplate omitted for clarity). It can be seen that membrane hasmoved out of plane of the plug thus opening a flow path through the holein the membrane for flow of air as illustrated by the arrow. FIG. 12cshows the situation where the membrane is deflected upwards. The size ofthe flow path, or acoustic conductance (or acoustic impedance) willdepend on the degree of deflection of the membrane 101 which in turndepends on the pressure difference across the membrane. Thus at lowpressures, with a relatively low membrane deflection, there will belimited opening of the flow path, and thus the presence of the vent willnot significantly impact on the operation of the transducer. At highpressure differentials such as resulting from high pressure events wherethere is significant membrane deflection there will be a significantopening of the vent and thus the pressure difference will rapidly reduceas described previously as the volumes on either side of the membraneequalise in pressure.

The extent of deflection required for the vent to open may be partlycontrolled by controlling the thickness of the plug section 1201. Asshown in FIG. 12 the plug section may have the same thickness as themembrane layer—this may ease manufacturing as will be described below.In other embodiments however the plug section may be thicker than themembrane layer so that the membrane must deflect by a large amount toclear the plug section and thus provide significant opening of the vent.

It will be appreciated that for air to flow when the membrane isdeflected downwards the supporting structure 1202 must not block thehole in the membrane 101. This may be achieved by ensuring that thesupport structure 1202 comprises one or more columns or pillars that hasa cross sectional area (in a plane parallel to the membrane 101) whichis less than that of the plug area, thus allowing air flow around thesupport columns. Additionally or alternatively however one or more holescould be provided through the support structure 1202. For example thesupport structure could comprise a frame having one or more open windowsto allow air to flow through the support structure.

In the embodiment shown in FIG. 12 where the plug section is supportedfrom the substrate 105 the relatively small gap between the membrane 101and substrate 101 may limit the amount of air flow from the volume underthe membrane. This may limit the maximum acoustic conductance of thevent even when largely open. Thus in some embodiments there may be oneor more channels or apertures 1204 extending from the volume defined bythe substrate 105 to the vicinity of the vent.

The structure shown in FIG. 12a may be manufactured using standardprocessing techniques with the addition of a few extra process steps. Asmentioned typically the membrane layer 101 is deposited on anappropriately shaped layer of sacrificial material which has beendeposited on the substrate. In one arrangement the support structurescould be fabricated in one or more deposition steps prior to depositingthe sacrificial material used to define the cavity under the membranelayer. The top of the support structure may be exposed at the top of thesacrificial material. Alternatively the sacrificial material may bedeposited and patterned to reveal spaces for the support structure.Material forming the support structure could then be deposited andetched appropriately so that the support structure extends through thesacrificial material where required. A frame structure could be providedby depositing the support structure and sacrificial material in severalsteps.

A layer of material such as silicon nitride suitable for the membranemay then be deposited on the sacrificial material and top of the supportstructures. A channel could then be etched to separate the membranelayer from the plug section. When subsequently the sacrificial materialis removed the membrane will be freely supported at the side walls to beflexible and the plug section will be supported to be largely fixed inposition. Any channel through the substrate may be etched as part of theback-etch process.

It will be appreciated that the presence of the support structure 1202on the substrate side of the membrane layer does mean that the flow pathwhen the membrane is defected downwards is not as large as the flow pathwhen the membrane is deflected upwards by a similar amount. Thus mayresult in a slight difference in response time of the vent to a givenpressure differential depending on which side is at high pressure.

FIG. 13 shows an alternative embodiment which is similar to that shownin FIG. 12 but wherein a plug section 1301 is supported from theback-plate 1302 by a support structure 1302. This structure will operatein a similar fashion to that described with reference to FIG. 12, buthas the advantages that the vent is located away from the substrate 105and thus there is no impediment to flow from the volume below thesubstrate and also the vent may be located in an area of greatermembrane deflection thus providing a larger flow path at high membranedeflections.

This structure may be fabricated in a similar way to that describedabove but with depositing the membrane layer and etching a channel toisolate plug section 1301 from the rest of the membrane layer prior toforming support structure 1302. Support structure 1302 may then beformed in a similar manner to that described above to provide a supportstructure extending through the sacrificial material used to define theupper cavity prior to depositing the material of the back-plate layer.

One or more transducers according to the any of the embodimentsdescribed above may be incorporated in a package. FIGS. 14a to 14gillustrate various different packaging arrangements. FIGS. 14a to 14geach show one transducer element located in the package but it will beappreciated that in some embodiments there may be more one thantransducer, e.g. a transducer array, and the various transducers may beformed on the same transducer substrate, i.e. a monolithic transducersubstrate, or may be formed as separate transducers with separatetransducer substrates each separate transducer substrate being bonded toa package substrate.

FIG. 14a shows a first arrangement where a transducer 1400 is located ina cover 1401, which forms at least part of a housing, on a packagesubstrate 1402. The cover in this example could be a metallic housingwhich is bonded to the substrate. The package substrate may comprise atleast one insulating layer. The package substrate may also comprise atleast one conductive layer. The package substrate may be a semiconductormaterial or may be formed from a material such as PCB, ceramic or thelike. Where the cover 1401 is metallic or itself comprises a conductivelayer the cover may be electrically coupled to the conductive layer ofthe substrate, e.g. so that the housing provides shielding forelectromagnetic interference (EMI). Bond wires 1403 may connect thetransducer to bond pads on the package substrate. In some embodiments,read-out circuitry, for instance amplifier circuitry, may be locatedwithin the housing formed in or connected to the package substrate.Through vias through the package substrate (not illustrated) may connectto contacts, i.e. solder pads, 1404 for electrically connecting externalcircuitry (not illustrated) to the package to allow transmission ofelectrical signals to/from the transducer 1400. In the example shown inFIG. 14a there is a sound port or acoustic port in the cover 1401 toallow sound to enter the package and the transducer is arranged in a topport arrangement.

FIG. 14b illustrates an alternative arrangement where the sound port isprovided in the package substrate 1402 and may, in use, be sealed. Aring 1405, which may be a sealing ring or a solder pad ring (for use informing a solder ring) may be provided around the periphery of the soundport on the outer side of the package to allow, in use, sealing of asound path leading to the sound port when the package is connected toanother PCB for example. In this embodiment the transducer is arrangedin a bottom port arrangement with the volume defined by the housing 1401forming part of the back-volume of the transducer.

FIG. 14c illustrates an example where instead of bond wires connectingthe transducer to the package substrate the transducer structure isinverted and flip-chip bonded to package substrate via connections 1406.In this example the sound port is in the package substrate such that thepackage is arranged in a bottom port arrangement.

FIG. 14d illustrates an alternative example to that of FIG. 14b whereina housing 1407 is formed from various panels of material, for examplePCB or the like. In this instance the housing 1407 may comprise one ormore conductive layers and/or one or more insulating layers. FIG. 14dshows the sound port in the package substrate. FIG. 14e shows analternative arrangement to that of FIG. 14b wherein a housing 1407 isformed from various panels of material, for example PCB or the like asdescribed in relation to FIG. 14d . FIG. 14f shows a further embodimentwhere the transducer structure is bonded via connections 1406 to thehousing upper layer, which may for instance be PCB or layeredconductive/insulating material. In this example however the electricalconnections to the package are still via contacts, solder pads, 1404 onthe package substrate, e.g. through vias (not illustrated) in thepackage substrate with conductive traces on the inside of the housing tothe transducer. FIG. 14g illustrates an alternative example to that ofFIG. 14c wherein a transducer is flip-chip bonded to the packagesubstrate in a housing 1407 formed from panels of material, for examplePCB or the like as described in relation to FIG. 14 d.

In general, as illustrated in FIG. 14h , one or more transducers may belocated in a package, the package is then operatively interconnected toanother substrate, such as a mother-board, as known in the art.

In all embodiments the variable vent may act as a non-linear vent, whichis a vent whose flow path size is not fixed and in which the extent towhich the vent is open, and also flow rate through the vent, varies withpressure differential in a non-linear way as described above.

Generally, embodiments of the invention therefore relate to a MEMStransducer comprising at least one variable vent. More specifically,embodiments of the invention therefore relate to a MEMS transducercomprising a transducer structure including a flexible membranesupported between a first volume and a second volume and at least onevariable vent structure. The variable vent structure may have at leastone moveable portion which is moveable in response to a high pressuredifferential across the moveable portion so as to provide a flow pathfor venting fluid, e.g. gas from at least one of said first and secondvolumes. The variable vent may therefore comprise an aperture where theopen size of the aperture varies with pressure differential.

The embodiments have been described in terms of venting air from avolume. The same principles apply to other gases and indeed otherfluids, possibly including liquids. In some embodiments the transducermay be arranged in a sealed environment which is filled with a fluidother than air, the sealed environment being arranged to allowtransmission of pressure waves to/from outside the sealed environment.There may still be large pressure differentials that can be generatedwithin the sealed environment and the use of variable vents in suchembodiments may be beneficial.

Embodiments of the present invention also relate to MEMS transducerscomprising a flexible membrane, and at least one variable vent structurewhich is substantially closed in a first range of pressure differentialsand which opens in a second higher range of pressure differentials toreduce the pressure differential across the membrane.

Embodiments of the present invention also relate to MEMS transducerscomprising a flexible membrane supported between a first volume and asecond volume and a vent structure connecting said first and secondvolumes. The vent provides a flow path having a size that varies withpressure differential across the membrane.

Embodiments of the present invention also relate to MEMS transducerscomprising a flexible membrane supported between a first volume and asecond volume and a vent connecting said first and second volumeswherein the vent is configured such that the flow rate through the ventis non-linear with respect to pressure difference.

Embodiments of the invention also relate to MEMS transducers having amembrane supported between first and second volumes wherein the acousticimpendence between the first and second volumes is variable with thedifferential pressure between the volumes.

Although the various embodiments describe a MEMS capacitive microphone,the invention is also applicable to any form of MEMS transducers otherthan microphones, for example pressure sensors or ultrasonictransmitters/receivers.

Embodiments of the invention may be usefully implemented using a rangeof different semiconductor-type material such a polysilicon for example.However the embodiments described herein are related to MEMS transducershaving membrane layers comprising silicon nitride.

It is noted that the embodiments described above may be used in a rangeof devices, including, but not limited to: analogue microphones, digitalmicrophones, pressure sensor or ultrasonic transducers. The inventionmay also be used in a number of applications, including, but not limitedto, consumer applications, medical applications, industrial applicationsand automotive applications. For example, typical consumer applicationsinclude portable audio players, laptops, mobile phones, tablets, PDAsand personal computers. Embodiments may also be used in voice activatedor voice controlled devices. Typical medical applications includehearing aids. Typical industrial applications include active noisecancellation. Typical automotive applications include hands-free sets,acoustic crash sensors and active noise cancellation.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Any reference signs in the claims shall not be construed so as to limittheir scope.

What is claimed is:
 1. A MEMS transducer comprising: a flexiblemembrane; and at least one variable vent structure wherein said variablevent structure provides a flow path between a first volume on one sideof the flexible membrane and a second volume on the other side of theflexible membrane, the vent structure having a size that variesaccording to a non-linear relationship with pressure differential acrossthe membrane.
 2. A MEMS transducer as claimed in claim 1, wherein saidvariable vent structure comprises at least one moveable portion which ismoveable in response to a pressure differential across the moveableportion so as to vary the size of a flow path through the vent structureand wherein the equilibrium position of the at least one moveableportion corresponds to a minimum size of flow path.
 3. A MEMS transduceras claimed in claim 2, wherein the at least one moveable portion isconfigured such that there is substantially no movement of the moveableportion from the equilibrium position at pressure differentials below afirst threshold.
 4. A MEMS transducer as claimed in claim 2, wherein thefirst threshold is greater than 150 Pa.
 5. A MEMS transducer as claimedin claim 2, wherein the first threshold is greater than 1 kPa.
 6. A MEMStransducer as claimed in claim 1, wherein at least one said variablevent structure provides substantially no significant variation in flowpath size for pressure differentials in the range of 0 Pa-200 Pa.
 7. AMEMS transducer as claimed in claim 2, wherein the at least one moveableportion is configured such that there is substantial movement of themoveable portion from the equilibrium position at pressure differentialsabove a second threshold.
 8. A MEMS transducer as claimed in claim 7,wherein the second threshold is lower than 100 kPa.
 9. A MEMS transduceras claimed in claim 1, wherein at least one said variable vent structureprovides substantially a significant increase in flow path size forpressure differentials in the range of 100 kPa-200 kPa, compared to theflow path size at equilibrium.
 10. A MEMS transducer as claimed in claim1, wherein at least one said variable vent structure comprises at leastone moveable portion which is moveable in response to a pressuredifferential across the moveable portion of at least 100 kPa.
 11. A MEMStransducer as claimed in claim 2, wherein the equilibrium position ofthe at least one moveable portion corresponds to the flow path beingsubstantially closed.
 12. A MEMS transducer as claimed in claim 1,wherein at least one said variable vent structure is formed in theflexible membrane and the flow path is a path through the flexiblemembrane.
 13. A MEMS transducer as claimed in claim 12, wherein the atleast one moveable portion comprises a portion of the flexible membranewhich is able to be deflected away from the surface of the rest of theflexible membrane.
 14. A MEMS transducer as claimed in claim 13, whereinsaid moveable portion of the flexible membrane is connected to the restof the flexible membrane via a beam structure.
 15. A MEMS transducer asclaimed in claim 14, wherein said beam structure is able to bend and/ortwist to allow said moveable portion of the flexible membrane to bedeflected away from the surface of the rest of the flexible membrane.16. A MEMS transducer as claimed in claim 1, wherein at least one saidvariable vent structure comprises at least two moveable portions of theflexible membrane, the at least two moveable portions being able to bedeflected away from the surface of the rest of the flexible membrane toexpose a hole in the flexible membrane.
 17. A MEMS transducer as claimedin claim 1, wherein at least one said variable vent structure is formedwith a flow-path that bypasses the flexible membrane.
 18. A MEMStransducer as claimed in claim 17, wherein the flow-path that bypassesthe flexible membrane runs through at least part of a sidewall of atransducer structure that supports the flexible membrane.
 19. A MEMStransducer as claimed in claim 1, comprising a back-plate structurewherein the flexible membrane is supported with respect to saidback-plate structure.
 20. A MEMS transducer as claimed in claim 1,wherein the transducer is a capacitive microphone.